Social interactions require quick perception, interpretation, and categorization of faces, with facial features offering cues to emotions, intentions, and traits. Importantly, reactions to faces depend not only on their features but also on their processing fluency, with disfluent faces suffering social devaluation. The current research used electrophysiological (EEG) and behavioral measures to explore at what processing stage and under what conditions emotional ambiguity is detected in the brain and how it influences trustworthiness judgments. Participants viewed male and female faces ranging from pure anger, through mixed expressions, to pure happiness. They categorized each face along the experimental dimension (happy vs. angry) or a control dimension (gender). In the emotion-categorization condition, mixed (ambiguous) expressions were classified relatively slower, and their trustworthiness was rated relatively lower. EEG analyses revealed that early brain responses are independent of the categorization condition, with pure faces evoking larger P1/N1 responses than mixed expressions. Some late (728- 880 ms) brain responses from central-parietal sites also were independent of the categorization condition and presumably reflect familiarity of the emotion categories, with pure expressions evoking larger central-parietal LPP amplitude than mixed expressions. Interestingly, other late responses were sensitive to both expressive features and categorization task, with ambiguous faces evoking a larger LPP amplitude in frontal-medial sites around 560-660 ms but only in the emotion categorization task. Critically, these late responses from the frontal-medial cluster correlated with the reduction in trustworthiness judgments. Overall, the results suggest that ambiguity detection involves late, top-down processes and that it influences important social impressions.

社交互动需要对面孔进行快速感知、解释和分类，面部特征可以提供情绪、意图和特征的线索。重要的是，对面孔的反应不仅取决于他们的特征，还取决于他们处理的流畅性，不合时宜的面孔会遭受社会贬低。目前的研究使用电生理学（EEG）和行为测量来探索大脑在什么处理阶段和什么条件下检测到情绪模糊性以及它如何影响可信度判断。参与者看到的男性和女性面孔从纯粹的愤怒、复杂的表情到纯粹的幸福。他们根据实验维度（快乐与愤怒）或对照维度（性别）对每张脸进行分类。在情感分类条件下，混合（模糊）表达的分类速度相对较慢，其可信度也相对较低。脑电图分析显示，早期大脑反应与分类条件无关，纯面部比混合表情更能引起更大的 P1/N1 反应。来自中央顶叶部位的一些晚期（728-880 ms）大脑反应也与分类条件无关，并且可能反映了对情绪类别的熟悉程度，纯表达比混合表达引起更大的中央顶叶 LPP 振幅。有趣的是，其他后期反应对表达特征和分类任务都很敏感，模糊的面孔在 560-660 毫秒左右的额叶内侧部位引起较大的 LPP 振幅，但仅在情感分类任务中。至关重要的是，这些来自额叶内侧簇的晚期反应与可信度判断的减少相关。总体而言，结果表明歧义检测涉及后期、自上而下的过程，并且它会影响重要的社会印象。